Applications for plasma torches in the prior art have generally focused on the use of DC arc plasma torches to process bulk solid wastes and to destroy toxic wastes. The emphasis has been on waste volume reduction and destruction efficiency. ICP torches have been used primarily in plasma spraying for surface preparation and in the production of special materials (metal oxides and carbides) in low volume. To date, the emphasis on the applications of ICP torches has been plasma gas dynamics and material interactions in the plasma jet. Neither prior art relating to DC arc plasma torches, nor prior art relating to ICP torches has focused on maximizing the electrical-to-thermal energy operating efficiency of the plasma source involved in a process, but rather, has focused on the unique process advantages offered by these devices. In addition to the well-known uses for plasma torches, such torches are also well suited to provide the thermal energy required to drive many chemical reactions, for example, those used to produce commercially valuable materials such as carbon monoxide (CO) and synthesis gas (a mixture of hydrogen (H.sub.2) and CO), and to also provide a highly excited reactant. The successful use of a chemically reactive plasma serving both as a reactant and as a source of heat to drive endothermic reactions for industrial applications requires good overall efficiency, with respect to both the operation of the plasma torch and the process that produces the desired product, to attain favorable process economics.
The two chemically reactive plasmas of primary interest for bulk thermal-chemical processes are steam and carbon dioxide (CO.sub.2). The use of steam as a chemically reactive plasma, and an ICP torch that employs a steam plasma are disclosed in U.S. Pat. No. 5,611,947. In this patent, superheated steam is generated and passed through an induction coil to produce a high temperature steam plasma usable for the conversion and disposal of various types of feedstock streams in a reactor vessel. While this prior art reference recognizes that total flow rate through the reactor vessel is a function of the plasma gas flow rate, it does not address the issue of optimizing the process with regard to the operation and efficiency of the plasma torch and the yield or conversion efficiency of the process.
ICP torch operating efficiencies can range from less than 10% to greater than 85%, depending upon the plasma gas type and the selected operating parameters for the torch. To encourage the application of ICP torches in a wider variety of industrial processes, it is desirable to maximize the operating efficiency of these devices. Bulk industrial processes typically require power levels in the megawatts range, thus operating efficiency is a key economic factor. Accordingly, a method is needed for maximizing the efficiency of an ICP torch by determining and maintaining an optimal plasma gas feed rate and power level. It has been determined that both of these parameters can greatly impact on the operating efficiency of an ICP torch. For example, the plasma gas flow rate can impact the torch efficiency by as much as 30-40% at a given power level.
As noted above, an ICP torch can be employed in producing CO and synthesis gas, and improving the efficiency of this process is also of importance in promoting the use of ICP torches. Synthesis gas is used as a chemical feedstock for the production of a wide variety of chemicals such as alcohols, aldehydes, acrylic acid, and ammonia. Several references detail the different uses of synthesis gas and the different methods used to produce it. Two such articles that are specifically incorporated herein by reference are: "Production of CO Rich Synthesis Gas," by Harold Gunardson and Joseph Abrardo, Air Products and Chemicals, Inc., Allentown, Pa., and "Advanced Reforming Technologies for Synthesis Gas Production" by Sandra Winter Madsen and Poul Rudberk of Haldor Tops.o slashed.e A/S, and Pierre Gauthier and Denis Cieutat of Air Liquide.
Several different processes are used conventionally to produce synthesis gas. Each process generates a different percentage mixture of H.sub.2 and CO. Standard practice in the industry is to express the synthesis mixture as the ratio of H.sub.2 to CO (H.sub.2 :CO). This ratio is very relevant in determining the kinds of products most appropriately produced from a particular synthesis gas. While there are methods to vary this ratio once the synthesis gas is produced, these ratio enhancement methods require additional investment in equipment and additional process steps.
Present commercial synthesis gas technology yields a product whose H.sub.2 :CO ratio varies from as high as 6:1 to as low as 3:2. There are some applications for synthesis gas in which excess H.sub.2 is desired, but more frequently CO is the more useful component of synthesis gas and thus, a lower ratio is more desirable. For example, renewed interest by the chemical industry in the Fisher-Tropsch process for synthesizing liquid fuels, such as gasoline, represents a potentially large market for a synthesis gas in which the H.sub.2 :CO ratio is about 2:1. Additionally, market studies show that the demand for CO is likely to increase dramatically over the next 10 years. It would be desirable to develop a method for easily and efficiently producing synthesis gas with a higher CO content, preferably having a ratio of 2/1 or less. It would further be desirable to easily and efficiently produce synthesis gas with an H.sub.2 :CO ratio of 1:1, or to produce a pure CO stream by using an ICP torch to treat a carbon feedstock rather than an organic feedstock.
Conventional processes for synthesis gas production that are capable of achieving low H.sub.2 :CO ratios typically do so by using a CO.sub.2 recycle technology in which a product gas has a CO.sub.2 impurity removed (CO.sub.2 is formed as a byproduct in conventional synthesis gas production as a result of oxidation reactions in the reaction vessel). The recovered CO.sub.2 is then re-injected into the reaction vessel, yielding a synthesis gas having a low ratio. While this technique produces synthesis gas having lower ratios, it involves additional process steps and expense. It would be preferable to achieve a low H.sub.2 :CO ratio without the need to utilize a CO.sub.2 recycle step in the process.
Conventional processes for producing synthesis gas are sensitive to contaminants in the feedstocks. For example, organic feedstocks often contain such high levels of sulfur that the sulfur must be removed prior to processing, because sulfur will poison the catalysts on which most commercial synthesis gas processes rely. Desulfurization involves additional process steps and expense. It would be desirable to produce synthesis gas from sulfur containing feedstocks without requiring pretreatment to remove the sulfur contaminant. An important aspect of this invention is a method that can easily produce synthesis gas without the need for the removal of contaminants such as sulfur from the feedstock.
Impurities are also introduced into the resultant synthesis gas stream in conventional processes as a byproduct of the reaction process. Steam reforming introduces H.sub.2 O vapor and CO.sub.2 that must be removed. Combustion-based reactions also introduce H.sub.2 O vapor and CO.sub.2, thus diluting the synthesis gas produced; and can also introduce nitrogen oxide (NO.sub.x) emissions and soot, which are contaminants requiring removal. Again, removal of these contaminants involves additional process steps and expense. It would therefore further be desirable to produce synthesis gas efficiently without the need to provide for the removal of diluents, such as H.sub.2 O vapor and CO.sub.2, or contaminants, such as NO.sub.x and soot.
Process parameters can be changed in conventional processes for synthesis gas production to enable the ratio of H.sub.2 :CO to be varied, but only over a relatively narrow range. Large-scale changes in the H.sub.2 :CO ratio require the additional steps of ratio enhancement and/or separation of CO from H.sub.2, representing added steps and expense. Furthermore, each specific conventional process to produce synthesis gas has a characteristic range of H.sub.2 :CO ratios that can be produced by that process. Before a synthesis gas production facility is constructed, it is critical to know what the desired H.sub.2 :CO ratio is, because the ratio desired would determine the process most suited to produce that ratio. Once the facility is constructed, adding ratio enhancement equipment to achieve different ratios is possible, but time consuming and expensive as well. Moreover, synthesis gas production facilities are often part of a larger petrochemical production facility, and the ratio of the synthesis gas required by such facilities can vary. It would be desirable to provide a method for producing synthesis gas capable of varying the H.sub.2 :CO ratio over a relatively wide range without the use of costly ratio enhancement techniques, so that synthesis gas production can be tailored to the varying needs of a site. The method should enable synthesis gas having a specific ratio to be produced simply by selectively introducing readily available reactants such as steam or CO.sub.2, along with an organic feed or by changing the organic feed. For example, if higher H.sub.2 :CO ratios are desired, steam in the form of a plasma and/or feed reactant can be introduced. If lower H.sub.2 :CO ratios are desired, carbon dioxide in the form of a plasma and/or feed reactant can be introduced.
Finally, many conventional methods to produce synthesis gas rely on reaction vessels that operate under high pressure. Such vessels are often more costly to build and operate than vessels that operate at much lower pressures. Furthermore, reactants can only be introduced into such high-pressure reaction vessels at the elevated operating pressure. Accordingly, it would be preferable to produce synthesis gas in a reaction vessel that operates at relatively low pressures so it is not necessary to supply the feedstock at a high pressure.